1. Field of the Invention
The present invention relates to an ion group irradiation device. The present invention also relates to a secondary ion mass spectrometer which analyzes an atom and a molecule that constitute the surface of a sample.
2. Description of the Related Art
Secondary ion mass spectroscopy (SIMS: Secondary Ion Mass Spectroscopy) is an analysis method of irradiating a sample with a primary ion beam, measuring a mass-to-charge ratio of a secondary ion emitted from the surface of a sample, and thereby specifying a species of the atom or a species of the molecule that constitutes the surface of the sample. In recent years, a method has received attention which specifies various kinds of molecules that constitute biomedical tissue and visualizes a fine two-dimensional distribution state of those molecules, by using SIMS.
In SIMS, secondary ions are emitted by a sputtering phenomenon which occurs by a collision between a primary ion and a molecule in the sample. Conventionally, a monatomic ion of gallium or the like has been used as the primary ion, but when a macromolecule such as a biomaterial has been determined to be a target, in particular, there has been a problem that detection sensitivity is poor, because when the monatomic ion is used, a molecular structure is remarkably decomposed. On the other hand, in recent years, a cluster ion of C60, bismuth, argon or the like is progressively used as the primary ion. The cluster ion contains a large number of atoms and molecules in one ion, and accordingly kinetic energy per one atom and one molecule is small. Accordingly, the decomposition of the molecular structure of the molecule in the sample can be alleviated. Furthermore, a sputtering yield by the cluster ion (number of atoms in sample, which are emitted per one primary ion) is high. Due to these effects, when the cluster ion is used, a yield of the secondary ion (number of secondary ions detected per one primary ion) in SIMS is enhanced, and in particular, the detection sensitivity for the secondary ion having a large mass is improved. However, the generation efficiency of the cluster ion is lower than that of the monatomic ion, and accordingly the number of the ions (corresponding to electric current value of ion) included in one irradiating ion group is generally small. For this reason, the amount of the secondary ions obtained by one time of irradiation with the ion group becomes very small, and in order to obtain a signal having sufficient intensity, it is necessary to enormously repeat the analysis cycle formed of irradiation with the primary ion and measurement of the secondary ion and integrate the data. From the above reasons, when the cluster ion is used in SIMS, there is a problem that a large amount of time periods is needed for the analysis for the sample.
The cluster ion can be obtained by jetting neutral particles of a material into a vacuum, generating neutral cluster particles by adiabatic expansion, and then ionizing the generated neutral cluster particles by an ionizing method such as electron bombardment. Here, the generation efficiency of the cluster ion can be enhanced and the average size of the cluster ion (number of atoms or molecules constituting the cluster ion) can be also increased by increasing a jet pressure to be applied when the material is jetted into the vacuum.
When the material is intermittently jetted with the use of an intermittent valve, the jet pressure per one time of jet can be increased, and accordingly the cluster ion having a large cluster size can be efficiently generated. Japanese Patent No. 3530942 discloses an apparatus which intermittently sprays a liquid material, desolvates the sprayed material to form a neutral cluster molecule, then photoionizes the neutral cluster molecule and thereby efficiently generates the cluster ion of the material.
On the other hand, a technology has been progressively developed which strictly limits the kinetic energy of the atom and the molecule which constitute the cluster ion, reduces the decomposition of the sample as much as possible and thereby secures the yield of the secondary ion. Japanese Patent Application Laid-Open No. 2011-29043 discloses an apparatus which controls the kinetic energy per one atom in the gas cluster ion to 20 eV or less.
There has been a problem in a conventional SIMS apparatus that the detection sensitivity is insufficient. When the cluster ion is used as the primary ion in order to solve this problem, it is necessary to enormously repeat the analysis cycle formed of irradiation with the primary ion and measurement of the secondary ion in order to obtain the mass spectrum and the mass distribution image of the secondary ions having sufficient intensity, and accordingly there has been a problem that a large amount of time periods is needed.
When the intermittent valve described in Japanese Patent No. 3530942 is used, the generation efficiency of the cluster can be improved. However, the operation (opening and closing) time cycle of the above described intermittent valve is as long as 20 msec to 1 sec, while the time cycle of the analysis cycle in a general SIMS apparatus is several hundreds psec, and accordingly when the intermittent valve is combined with SIMS, the operation cycle of the intermittent valve becomes the rate-limiting factor of the time cycle of the analysis cycle. From the above description, even if the efficiency of cluster generation is improved and the repeat count of the cycles can be reduced due to the use of the intermittent valve, the total time period necessary for all the repeat counts cannot be still shortened.
When the apparatus described in Japanese Patent Application Laid-Open No. 2011-29043 is used, the decomposition of a molecule in the sample can be suppressed, and accordingly a high yield of the secondary ion can be secured. However, because the gas is continuously jetted, it is difficult for the jet pressure to be raised, from the viewpoint of keeping a vacuum. As a result, the efficiency of cluster generation is not enhanced, and the amount of the secondary ions which are obtained by one time of irradiation with the ion group is still very small. Accordingly, even if the apparatus in Japanese Patent Application Laid-Open No. 2011-29043 is used, an enormous number of integrations is needed, and accordingly an analysis time period cannot be still shortened.